Many condition control devices, and particularly thermostats, operate by opening or closing an electrical circuit having two wires connected to a load and a source of potential. The use of electromechanical types of condition responsive equipment, such as bimetal operated thermostats, easily utilized the two-wire control concept since no electric energy was drawn from the two control wires. The wires were used solely for completing an electric circuit to the load and a source of power.
In more recent years electronic types of condition control devices and thermostats have been developed. In this type of a control device or thermostat, the electronics within the device require electric power and this power has been typically supplied by a pair of additional wires to the control unit. This made the retrofitting of an electronic type of condition control device or thermostat for a mechanical thermostat a difficult, and sometimes prohibitively expensive retrofit project.
In recent years the substitution of electronically controlled thermostats for mechanically operated thermostats has accelerated because of the desirability and need for clock operated night setback and day setup functions. In certain prior art devices a battery has been placed in the thermostat to operate the electronics and the timing function, and this has led to the usual difficulties where a battery powered device is involved.
In certain prior art devices these problems have been recognized and overcome by placing a special transformer in the thermostat. The transformer was a current and voltage type of transformer. The transformer supplied power to the electronics through the voltage winding when the thermostat was not completing an electric circuit to control an external load, while supplying energy to the electronics through the current winding of the transformer when the circuit was completed to operate the external load. This added to the size and complexity, as well as the cost, of the thermostat by the addition of a very specialized type of current and voltage transformer.